Deformable mechanical bi-stable devices (“bi-stable” is defined herein as referring to such devices having two or more stable positions, and therefore includes multi-stable devices) have been widely used for a variety of applications. For example, user input devices such as push-buttons are commonly used in many devices and may utilize a bi-stable device where each position of the button corresponds to a stable position of the bi-stable device. Computers, telephones, and vehicle control panels are just a few of the numerous applications requiring some sort of user input that can utilize a bi-stable device. Another application for a bi-stable device is for a hinge where a bi-stable device is incorporated into a hinge mechanism to provide stop points or resting positions for the hinged components, each such stop point or resting position corresponding to a stable position of the bi-stable device.
An exemplary bi-stable device, which can be used for illustration of characteristics shared by a wide variety of bi-stable devices, is known as the oil can device, and is illustrated in FIGS. 1A and 1B. FIGS. 1A and 1B depict a schematic diagram of an exemplary bi-stable device 10 having a circular deformable panel 14. In FIGS. 1A and 1B, a cylindrical mounting member 12 having an upper lip portion 13 is mounted on a support structure 11. An elastically deformable circular panel 14 is attached to the inner circumference of the mounting member. The elastically deformable panel would have a normally flat state, but is sized to have a diameter in its normal flat state that is greater than the internal diameter of the mounting member so that when it is mounted in the mounting member it is placed under a force load along vectors between opposing points on the circumference of the panel (in the horizontal plane as shown in FIG. 1). This load causes the elastically deformable panel to deform into one of two stable states, described for sake of convenience as an upper or first stable position depicted in FIG. 1A and a lower or second stable position depicted in FIG. 1B. The panel thus acts as a bi-stable snap-action panel, deformable between a convex stable configuration and a concave stable configuration. This configuration is also sometimes referred to as the “oil-can” configuration because the bi-stable snap action deformation was used in traditional old-style oil cans to displace oil out of an opening in the can.
One way that the performance characteristics of a bi-stable device is commonly represented is by a plot of potential energy versus position (i.e., physical displacement) of the device. FIG. 2 depicts a potential energy plot for an exemplary bi-stable device such as the device of FIG. 1, with potential energy E represented on the y-axis and position (P) represented on the x-axis. As seen in FIG. 2, the two stable positions of the device from FIG. 1 (corresponding to the respective positions illustrated in FIGS. 1A and 1B) are shown at positions P1 and P2 where the potential energy as at its minimum Emin. The position P1 represents the potential energy at the unstable position of maximum potential energy Emax when the deformable panel 14 passes through the position mid-way between the positions of FIGS. 1A and 1B where it is neither convex nor concave. It should be noted that the exemplary device of FIGS. 1A and 1B has symmetrically stable positions that have the same potential energy, Emin. Other device designs may have unsymmetrical stable positions (or multiple stable positions) that have different free energies.
Two important performance characteristics of the bi-stable device performance represented in FIG. 2 are transition energy and stability. Transition energy is the amount of energy required to transition the bi-stable device from one stable position (P1) to the other stable position (P2), and is represented by the difference between Emin and Emax. “Stability” refers to the stability of the bi-stable device in its stable positions, and is represented by the steepness of the slope of curve of the potential energy plot on either side of the stable positions P1 and P2. A relatively steep slope on either side of the stable positions represents a relatively high level of stability, whereas a relatively shallow slope on either side of the stable positions represents a relatively low level of stability.
It is often desirable in certain situations to provide a bi-stable device with a relatively low transition energy so that the device is easier to move between stable positions. This can be accomplished by reducing the stiffness of the deformable member, either by using a material with a lower Young's modulus, by reducing the thickness of the deformable member, or by increasing the length (between mounting members) or diameter of the deformable member, or by any combination of these factors. Another way the transition energy can be modulated is by utilizing an elastic member such as a spring in conjunction with the deformable member, such as shown in FIG. 3. The pre-load in the spring 22 can be adjusted to modify the transition energy required to switch the deformable member 14 from one stable position to another. In FIG. 3, the deformable panel 14 is square or rectangular in shape, and is mounted and disposed between fixed right mounting member 12 and sliding left mounting member 20. Slidable right mounting member 20 is slidably mounted on support structure 11. Fixed right mounting member 12 and left member 12′ are each fixedly mounted on the support structure 12. Spring 22 is disposed between and connected to mounting member 12 and slidable mounting member 20. Spring 22 is configured so that it is in an expanded state, and thus urges slidable member 20 towards mounting member 12, thereby exerting a horizontal compressive load on the deformable panel 14 to cause it to deform into one of two stable positions. During deformation of the deformable panel 14, slidable member 20 moves to the left as spring 22 absorbs energy, thereby reducing the amount of force required to displace the deformable panel.
The energy plot of an exemplary device like the one shown in FIG. 3 (or the FIG. 1 device where a lower modulus used) is shown in FIG. 4. As can be seen from a comparison of FIG. 4 with FIG. 2, the transition energy is significantly lower in FIG. 4 (note that although FIGS. 2 and 4 are not drawn to a particular scale, they are proportionately dimensioned for purposes of visual comparison). However, reducing the transition energy for a bi-stable device in this fashion where the stable positions P1 and P2 are kept the same also results in a reduction in stiffness of the system around its stable positions. The reduction in stiffness is readily observed by comparing the curves shown in FIGS. 2 and 4. The stiffness of the system at (any point, but specifically at) a stable position varies as the curvature (or varies inversely as the radius of curvature) of the energy vs. displacement graph. Higher curvature (or lower radius of curvature) indicates higher system stiffness at the stable position. Note that curvature is a local property i.e. it is strictly defined only at a point. Theoretically, the slope of the sides of the ‘W’ shaped energy vs. displacement graph is not relevant to the stiffness of the system precisely at the stable position. Of course, practical systems have smooth energy vs. displacement graphs, which necessitates that graphs with a high curvature at a stable position will also have steep slopes near this position. The steepness of the slope of the plot on either side of the stable positions P1 and P2, where it is seen that the steepness of the slope is much lower for FIG. 4 than for FIG. 2, indicating lower stiffness. Lower stiffness is often undesirable as the device may be displaced away from its stable position by a considerable distance by unintended vibration or incidental contact; however, conventional techniques have been unable to reduce transition energy without also reducing stiffness.
Accordingly, there is a need for bi-stable devices that can provide low levels of transition energy with high levels of stiffness in their stable positions.